Photolithography plays a critical role in the art of printed circuit packaging. Photolithography is used to define in a thin film of photoresist those regions from which copper is to be selectively etched to subtractively form circuitization, or to which copper is selectively plated to additively form circuitization. Photolithography is also used to personalize soldermasks and dielectric layers.
There are basically two types of photoresist: negative acting and positive acting. Positive photoresists and negative photoresists are both formed from monomers, hereinafter referred to as "monomeric units", such as, for example, acrylates and ethers of bisphenol A. Examples of monomeric units used in the conventional photoresists are as follows: t-butyl acrylate, 1, 5 pentanediol diacrylate, N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate, 1,4-butanediol diacrylate, diethylene glycol diacrylate, hexamethylene glycol diacrylate, 1,3-propanediol diacrylate, decamethylene glycol diacrylate, decamethylene glycol dimethacrylate, 1,4-cyclohexanediol diacrylate, 2,2-dimethylolpropane diacrylate, glycerol diacrylate, tripropylene glycol diacrylate, glycerol triacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyoxyethylated trimethylolpropane triacrylate and trimethacrylate and similar compounds as disclosed in U.S. Pat. No. 3,380,831, 2,2-di-(p-hydroxyphenyl)-propane diacrylate, pentaerythritol tetraacrylate, 2,2-di(p-hydrohyphenyl)-propane dimethacrylate, triethylene glycol diacrylate, polyoxyethyl-2,2-di(p-hydroxyphenyl)-propane dimethacrylate, di-(3-methacryloxy-2-hydroxypropyl) ether of bisphenol-A, di-(2-methacryloxyethyl) ether of bisphenol-A, di-(3-acryloxy-2-hydroxypropyl) ether of bisphenol-A, di-(2-acryloxyethyl) ether of bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of tetrachloro-bisphenol-A, di-(2-methacryloxyethyl) ether oftetrachloro-bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) etheroftetrabromo-bisphenol-A, di-(2-methacryloxyethyl) ether of tetrabromo-bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of 1,4-butanediol, di-(3-methacryloxy-2-hydroxypropyl) ether of diphenolic acid, triethylene glycol dimethacrylate, polyoxypropyltrimethylol propane triacrylate, ethylene glycol dimethacrylate, butylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetriol trimethacrylate, 2,2,4-trimethyl-1,3-pentanediol dimethacrylate, pentaerythritol trimethacrylate, 1-phenyl ethylene-1,2-dimethacrylate, pentaerythritol tetramethacrylate, trimethylol propane trimethacrylate, 1,5-pentanediol dimethacrylate, diallyl fumarate, styrene, 1,4-benzenediol dimethacrylate, 1,4-diisopropenyl benzene, and 1,3,5-triisopropenyl benzene.
In addition to the reactive monomeric units mentioned above, the photoimageable compositions used to form negative and positive photoresists can also contain one or more free radical-initiated and polymerizable species with molecular weight of at least about 300. Monomeric units of this type are an alkylene or a polyalkylene glycol diacrylate and those described in U.S. Pat. No. 2,927,022. Particulate thickeners such as, for example, silicas, clays, alumina, bentonites, kalonites, and the like can also be used in the photoimageable compositions. Dyes and pigments may also be added to increase the visibility of the resist image. Any colorant used however, should be transparent to the actinic radiation used to polymerize the monomeric units. With some compositions, it is desirable to add a plasticizer, either solid or liquid, to give flexibility to the film or coating. Generally inert solvents which are volatile at ordinary pressures are used to prepare these photoresist compositions.
Photoimageable compositions are also used to form soldermasks. Such compositions typically comprise acrylate monomers or epoxy monomers, free radical initiators, and thermal cross-linking agents. A photoimageable soldermask composition which contains acrylate monomers and a free radical initiator is described in U.S. Pat. No. 5,026,624 assigned to the assignee of the present application, disclosure of which is incorporated herein by reference. A photoimageable soldermask composition, which contains cationically polymerizable epoxy monomers and cationic photoinitiators, is described in U.S. Pat. No. 5,300,402.
During processing of the photoresist or soldermask, an acrylate-based or epoxy-based photoimageable film is first applied to a circuit board and then patterned by exposure of preselected regions to actinic radiation. To develop the resulting pattern of polymerized and unpolymerized material, the coated board is contacted with a liquid developer either by dipping or spraying. The commonly assigned U.S. Pat. No. 5,268,260 of N. R. Bantu, Anilkumar Bhatt, Ashwinkumar Bhatt, G. W. Jones, J. A. Kotylo, R. F. Owen, K. I. Papathomas, and A. K. Vardya for Photoresist Develop and Strip Solvent Compositions and Method for Their Use, incorporated herein by reference, describes the use of the low vapor pressure, high boiling solvents benzyl alcohol, gamma butyrolactone, and propylene carbonate for developing acrylate-based photoresist such as DuPont Riston T-168 and solvent-processable soldermasks such as the Vacrel 700 and 900 series. In the case of negative acting photoresists, the unpolymerized material used to form the photoresist, i.e., the monomeric units of acrylate or epoxy, is dissolved in the developer at low temperature, preferably between 15.degree. C. and 45.degree. C. Developing of the soldermask involves the same steps except that the unpolymerized material is dissolved in the developer at a temperature of from about 15.degree. C. to about 80.degree. C. Positive acting resists behave oppositely. Actinic radiation renders the positive acting photoresist more soluble in the developer, and the exposed regions are removed preferentially by the developer.
The dissolved material, which consists primarily of monomeric units of the acrylate or epoxy, and the developing solution are then removed from the board by allowing the solution to run off into a containment tank. To further enhance development of the pattern, the residual dissolved materials and developing solution are rinsed from the board, preferably with warm water. High vapor pressure organic solvents, such as isopropyl alcohol, acetone, methyl ethyl ketone and xylene, may also be used as a rinse. The effluent produced by this process is an impure solution of developer, which is laden with monomeric units and other impurities. Typically, the effluent contains greater than 10 percent by weight of monomeric units.
Large volumes of liquid waste containing impure benzyl alcohol, impure propylene carbonate, or impure gamma butyrolactone result from the above-described developing process. This liquid waste must be further processed prior to release into the environment (as by incineration). Such methods of dealing with the liquid waste are costly. Moreover, the costs to the industry in terms of purchasing virgin material, i.e., pure benzyl alcohol, pure gamma butyrolactone, and pure propylene carbonate and the costs to the environment of manufacturing virgin material are significant.
Accordingly, it is desirable to have a new method of reducing the amount of solvent containing waste that results from this and other industrial processes. A method that permits recovery of relatively pure solvent which can then be re-used by the industry, and thus prevent the need to purchase virgin material, is especially desirable.